Bottom Line:
Although they are classically activated by immunoglobulin (Ig)E antibodies, a unique property of mast cells is their antibody-independent responsiveness to a range of cationic substances, collectively called basic secretagogues, including inflammatory peptides and drugs associated with allergic-type reactions.The pathogenic roles of these substances have prompted a decades-long search for their receptor(s).Finally, we determine that Mrgprb2 and MRGPRX2 are targets of many small-molecule drugs associated with systemic pseudo-allergic, or anaphylactoid, reactions; we show that drug-induced symptoms of anaphylactoid responses are significantly reduced in knockout mice; and we identify a common chemical motif in several of these molecules that may help predict side effects of other compounds.

ABSTRACTMast cells are primary effectors in allergic reactions, and may have important roles in disease by secreting histamine and various inflammatory and immunomodulatory substances. Although they are classically activated by immunoglobulin (Ig)E antibodies, a unique property of mast cells is their antibody-independent responsiveness to a range of cationic substances, collectively called basic secretagogues, including inflammatory peptides and drugs associated with allergic-type reactions. The pathogenic roles of these substances have prompted a decades-long search for their receptor(s). Here we report that basic secretagogues activate mouse mast cells in vitro and in vivo through a single receptor, Mrgprb2, the orthologue of the human G-protein-coupled receptor MRGPRX2. Secretagogue-induced histamine release, inflammation and airway contraction are abolished in Mrgprb2- mutant mice. Furthermore, we show that most classes of US Food and Drug Administration (FDA)-approved peptidergic drugs associated with allergic-type injection-site reactions also activate Mrgprb2 and MRGPRX2, and that injection-site inflammation is absent in mutant mice. Finally, we determine that Mrgprb2 and MRGPRX2 are targets of many small-molecule drugs associated with systemic pseudo-allergic, or anaphylactoid, reactions; we show that drug-induced symptoms of anaphylactoid responses are significantly reduced in knockout mice; and we identify a common chemical motif in several of these molecules that may help predict side effects of other compounds. These discoveries introduce a mouse model to study mast cell activation by basic secretagogues and identify MRGPRX2 as a potential therapeutic target to reduce a subset of drug-induced adverse effects.

Mentions:
Next, we determined whether MrgprB2 is the basic secretagogue receptor in mouse mast cells. The MrgprB2 genomic locus contains too much repetitive sequence to permit gene targeting through homologous recombination (Extended Data Fig. 5a). Therefore, we used a zinc finger nuclease-based strategy to generate a mouse line with a 4 base pair deletion in the MrgprB2 coding region (MrgprB2MUT mice), resulting in a frameshift mutation and early termination shortly after the first transmembrane domain (Extended Data Fig. 5b-d). The mutation was stable and inheritable (Extended Data Fig. 5c), so we regard MrgprB2MUT as a functional . Mast cell numbers were comparable in tissues of wild-type (WT) and MrgprB2MUT mice, indicating that MrgprB2 is not essential for mast cell survival or targeting to tissue (Extended Data Fig. 6a). Responsiveness of peritoneal mast cells to anti-IgE antibodies (Fig. 2a) and endothelin (Extended Data Fig. 7) also was comparable, demonstrating that MrgprB2 mutation does not globally impair IgE or GPCR-mediated mast cell signaling. However, 48/80-induced mast cell activation (Fig. 2a) and tissue histamine release essentially was abolished in mutant mast cells (Fig. 2b; Extended Data Fig. 6b). Further, we found that 48/80-evoked tracheal contraction (Fig. 2c) and hindpaw inflammation (extravasation and swelling; Fig. 2d) were almost completely absent in an MrgprB2MUT background, while antigen (Fig. 2c) and anti-IgE evoked responses (Extended Data Fig. 8) were comparable to WT mice. Finally, we found that four additional basic secretagogues, as well as MrgprX2 agonists PAMP (9-20) and cortistatin10, strongly activated WT but not MrgprB2MUT mast cells (Fig. 2e; Extended Data Fig. 9a). HEK293 cells expressing MrgprB2 or MrgprX2 (MrgprX2-HEK) also responded to these secretagogues (Extended Data Fig. 2). Taken together, we conclude that MrgprB2 is the mouse mast cell basic secretagogue receptor. It is likely that the list of small, basic peptides that activate MrgprB2 is greater than the number in this study; indeed, dozens of such peptides have been shown to activate mast cells3,6,16,17. Notably, human MrgprX2 is much more sensitive to Substance P than mouse MrgprB2 (Extended Data Fig. 2c), suggesting a potential species-specific role for Substance P in mast cell signaling.

Mentions:
Next, we determined whether MrgprB2 is the basic secretagogue receptor in mouse mast cells. The MrgprB2 genomic locus contains too much repetitive sequence to permit gene targeting through homologous recombination (Extended Data Fig. 5a). Therefore, we used a zinc finger nuclease-based strategy to generate a mouse line with a 4 base pair deletion in the MrgprB2 coding region (MrgprB2MUT mice), resulting in a frameshift mutation and early termination shortly after the first transmembrane domain (Extended Data Fig. 5b-d). The mutation was stable and inheritable (Extended Data Fig. 5c), so we regard MrgprB2MUT as a functional . Mast cell numbers were comparable in tissues of wild-type (WT) and MrgprB2MUT mice, indicating that MrgprB2 is not essential for mast cell survival or targeting to tissue (Extended Data Fig. 6a). Responsiveness of peritoneal mast cells to anti-IgE antibodies (Fig. 2a) and endothelin (Extended Data Fig. 7) also was comparable, demonstrating that MrgprB2 mutation does not globally impair IgE or GPCR-mediated mast cell signaling. However, 48/80-induced mast cell activation (Fig. 2a) and tissue histamine release essentially was abolished in mutant mast cells (Fig. 2b; Extended Data Fig. 6b). Further, we found that 48/80-evoked tracheal contraction (Fig. 2c) and hindpaw inflammation (extravasation and swelling; Fig. 2d) were almost completely absent in an MrgprB2MUT background, while antigen (Fig. 2c) and anti-IgE evoked responses (Extended Data Fig. 8) were comparable to WT mice. Finally, we found that four additional basic secretagogues, as well as MrgprX2 agonists PAMP (9-20) and cortistatin10, strongly activated WT but not MrgprB2MUT mast cells (Fig. 2e; Extended Data Fig. 9a). HEK293 cells expressing MrgprB2 or MrgprX2 (MrgprX2-HEK) also responded to these secretagogues (Extended Data Fig. 2). Taken together, we conclude that MrgprB2 is the mouse mast cell basic secretagogue receptor. It is likely that the list of small, basic peptides that activate MrgprB2 is greater than the number in this study; indeed, dozens of such peptides have been shown to activate mast cells3,6,16,17. Notably, human MrgprX2 is much more sensitive to Substance P than mouse MrgprB2 (Extended Data Fig. 2c), suggesting a potential species-specific role for Substance P in mast cell signaling.

Bottom Line:
Although they are classically activated by immunoglobulin (Ig)E antibodies, a unique property of mast cells is their antibody-independent responsiveness to a range of cationic substances, collectively called basic secretagogues, including inflammatory peptides and drugs associated with allergic-type reactions.The pathogenic roles of these substances have prompted a decades-long search for their receptor(s).Finally, we determine that Mrgprb2 and MRGPRX2 are targets of many small-molecule drugs associated with systemic pseudo-allergic, or anaphylactoid, reactions; we show that drug-induced symptoms of anaphylactoid responses are significantly reduced in knockout mice; and we identify a common chemical motif in several of these molecules that may help predict side effects of other compounds.

ABSTRACTMast cells are primary effectors in allergic reactions, and may have important roles in disease by secreting histamine and various inflammatory and immunomodulatory substances. Although they are classically activated by immunoglobulin (Ig)E antibodies, a unique property of mast cells is their antibody-independent responsiveness to a range of cationic substances, collectively called basic secretagogues, including inflammatory peptides and drugs associated with allergic-type reactions. The pathogenic roles of these substances have prompted a decades-long search for their receptor(s). Here we report that basic secretagogues activate mouse mast cells in vitro and in vivo through a single receptor, Mrgprb2, the orthologue of the human G-protein-coupled receptor MRGPRX2. Secretagogue-induced histamine release, inflammation and airway contraction are abolished in Mrgprb2- mutant mice. Furthermore, we show that most classes of US Food and Drug Administration (FDA)-approved peptidergic drugs associated with allergic-type injection-site reactions also activate Mrgprb2 and MRGPRX2, and that injection-site inflammation is absent in mutant mice. Finally, we determine that Mrgprb2 and MRGPRX2 are targets of many small-molecule drugs associated with systemic pseudo-allergic, or anaphylactoid, reactions; we show that drug-induced symptoms of anaphylactoid responses are significantly reduced in knockout mice; and we identify a common chemical motif in several of these molecules that may help predict side effects of other compounds. These discoveries introduce a mouse model to study mast cell activation by basic secretagogues and identify MRGPRX2 as a potential therapeutic target to reduce a subset of drug-induced adverse effects.